Why does the plane stay in the air? School encyclopedia. Pilot training: theory first

The arrival of summer in some hot corners of our planet brings with it not only sweltering heat, but also flight delays at airports. For example, in Phoenix, Arizona, the air temperature recently reached +48°C and airlines were forced to cancel or reschedule over 40 flights. What is the reason? Don't planes fly when it's hot? They fly, but not at any temperature. According to media reports, heat poses a particular problem for Bombardier CRJ aircraft, which have a maximum takeoff operating temperature of +47.5°C. In the same time, large planes from Airbus and Boeing can fly at temperatures up to +52°С degrees or so. Let's figure out what causes these restrictions.

Lift principle

Before explaining why not every aircraft is able to take off at high air temperatures, it is necessary to understand the very principle of how airplanes fly. Of course, everyone remembers the answer from school: “It’s all about the lift of the wing.” Yes, this is true, but not very convincing. To really understand the laws of physics that are involved here, you need to pay attention to law of momentum. In classical mechanics, the momentum of a body is equal to the product of the mass m of this body and its speed v, the direction of the momentum coincides with the direction of the velocity vector.

At this point, you might think that we are talking about a change in the airplane's momentum. No, instead consider the change in air momentum, impinging on the plane of the wing. Imagine that each air molecule is a tiny ball that collides with an airplane. Below is a diagram that shows this process.

A moving wing collides with balloons(that is, air molecules). The balls change their momentum, which requires the application of force. Since action equals reaction, the force that the wing exerts on the air pellets is the same magnitude as the force that the pellets themselves exert on the wing. This leads to two results. Firstly, the lifting force of the wing is provided. Secondly, a reverse force appears - thrust. You can't achieve lifting without traction..

To generate lift, the plane must move, and to increase its speed, you need more thrust. To be more precise, you need just enough thrust to balance the force of air resistance - then you fly at the speed you want. Typically, this thrust is provided by a jet engine or propeller. Most likely, you could even use a rocket engine, but in any case, you need a thrust generator.

What does the temperature have to do with it?

If the wing hits just one ball of air (that is, a molecule), it will not produce much lift. To increase lift, you need a lot of collisions with air molecules. This can be achieved in two ways:

move faster, increasing the number of molecules that come into contact with the wing per unit time;
design wings with larger surface area, because in this case the wing will collide with a large number of molecules;
Another way to increase the contact surface area is to use greater angle of attack due to the tilt of the wings;
finally, it is possible to achieve a greater number of collisions between the wing and air molecules if the density of the air itself is higher, that is, the number of molecules themselves per unit volume is greater. In other words, increasing air density increases lift.

This conclusion brings us to air temperature. What is air? These are many microparticles, molecules that move right around us in different directions and at different speeds. And these particles collide with each other. As the temperature rises average speed the movement of molecules also increases. An increase in temperature leads to expansion of the gas, and at the same time - to a decrease in air density. Remember that heated air is lighter than cold air; the principle of hot air balloon aeronautics is based on this phenomenon.

So, for more lift you need either higher speed or big square wing, or a greater angle of attack of molecules on the wing. Another condition: the higher the air density, the greater the lifting force. But the opposite is also true: the lower the air density, the lower the lift. And this is true for hot parts of the planet. Because of high temperature air density is too low for some aircraft, it is not enough for them to take off.

Of course, you can compensate for the decrease in air density by increasing the speed. But how can this be done in reality? In this case, it is necessary to install more powerful engines on the aircraft, or increase the length of the runway. Therefore, it is much easier for airlines to simply cancel some flights. Or, at least, move it to the evening, early morning, when the ambient temperature is below the maximum permissible limit.

Why do birds fly?

A bird's wing is designed to create a force that counteracts the force of gravity. After all, a bird's wing is not flat, like a board, but curved . This means that the air stream flowing around the wing must travel a longer path along the upper side than along the concave lower side. In order for both air flows to reach the tip of the wing at the same time, the air flow above the wing must move faster than under the wing. Therefore, the speed of air flow over the wing increases and the pressure decreases.

The difference in pressure under and above the wing creates a lift force directed upward and counteracting the force of gravity.

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Airplanes are very complex devices, sometimes frightening with their complexity to ordinary people, people unfamiliar with aerodynamics.

The mass of modern airliners can reach 400 tons, but they remain calm in the air, move quickly and can cross enormous distances.

Why does the plane fly?

Because he, like a bird, has a wing!

If the engine fails, it’s okay, the plane will fly on the second one. If both engines failed, history knows of cases where even in such circumstances they landed. Chassis? Nothing prevents the plane from landing on its belly; if certain fire safety measures are observed, it will not even catch fire. But a plane can never fly without a wing. Because it is what creates lift.

Airplanes continuously “run” into the air with their wings, set at a slight angle to the air flow velocity vector. This angle is called the "angle of attack" in aerodynamics. The "angle of attack" is the angle of inclination of the wing to the invisible and abstract "flow velocity vector". (see figure 1)

Science says that a plane flies because a zone of increased pressure is created on the lower surface of the wing, due to which an aerodynamic force appears on the wing, directed upwards perpendicular to the wing. For the convenience of understanding the flight process, this force is decomposed according to the rules of vector algebra into two components: the aerodynamic drag force X

(it is directed along the air flow) and the lift force Y (perpendicular to the air velocity vector). (see figure 2)

When creating an aircraft, great attention is paid to the wing, because the safety of flights will depend on it. Looking out the window, the passenger notices that it is bending and is about to break. Don't be afraid, it can withstand enormous loads.

In flight and on the ground, the aircraft's wing is “clean”; it has minimal air resistance and sufficient lift to keep the aircraft at altitude while flying at high speed.

But when the time comes for takeoff or landing, the plane needs to fly as slowly as possible so that on one side the lift does not disappear, and on the other the wheels can withstand touching the ground. To achieve this, the wing area is increased: flaps(plane at the back) and slats(at the front of the wing).

If you need to further reduce the speed, then in the upper part of the wing they are released spoilers, which act as an air brake and reduce lift.

The plane becomes like a bristling beast, slowly approaching the ground.

Together: flaps, slats and spoilers- called wing mechanization. Pilots release the mechanization manually from the cockpit before takeoff or landing.

This process usually involves a hydraulic system (less often an electric one). The mechanism looks very interesting and is at the same time very reliable.

On the wing there are steering wheels (in aviation ailerons), similar to those on a ship (it’s not for nothing that a plane is called an aircraft), which deviate, tilting the plane in the desired direction. They usually deflect synchronously on the left and right sides.

Also on the wing there are aeronautical lights , which are designed to ensure that from the side (from the ground or another aircraft) it is always visible in which direction the aircraft is flying. The fact is that red is always on on the left, and green on the right. Sometimes white “flashing lights” are placed next to them, which are very visible at night.

Most of the aircraft's characteristics directly depend on the wing, its aerodynamic quality and other parameters. Fuel tanks are located inside the wing (the maximum volume of fuel to be filled depends very much on the size of the wing), electric heaters are installed on the leading edge so that ice does not build up there in the rain, landing gear is attached to the root part...

The aircraft speed is reached using a power plant or turbine. Due to the power plant that creates traction force, the aircraft is able to overcome air resistance.

Airplanes fly according to the laws of physics

Aerodynamics as a science is based on theorem of Nikolai Egorovich Zhukovsky, an outstanding Russian scientist, the founder of aerodynamics, which was formulated back in 1904. A year later, in November 1905, Zhukovsky outlined his theory of creating the lifting force of an aircraft wing at a meeting of the Mathematical Society.

Why do planes fly so high?

Flight altitude of modern jet aircraft is within from 5000 to 10000 meters above sea level. This can be explained very simply: at such an altitude, the air density is much lower, and, therefore, air resistance is lower. Airplanes fly at high altitudes because when flying at an altitude of 10 kilometers, the aircraft consumes 80% less fuel than when flying at an altitude of one kilometer.

However, why then do they not fly even higher, in the upper layers of the atmosphere, where the air density is even less?

The fact is that to create the necessary thrust by an aircraft engine a certain minimum air supply is required. Therefore, every aircraft has a maximum safe flight altitude, also called a “service ceiling.” For example, the service ceiling of the Tu-154 aircraft is about 12,100 meters.

There is probably no person who, watching a plane fly, has not wondered: “How does it do it?”

People have always dreamed of flying. Icarus can probably be considered the first aeronaut who tried to take off with the help of wings. Then, over the millennia, he had many followers, but the real success fell to the lot of the Wright brothers. They are considered the inventors of the airplane.

Seeing huge passenger airliners on the ground, double-decker Boeings, for example, it is completely impossible to understand how this multi-ton metal colossus rises into the air, it seems so unnatural. Moreover, even people who have worked all their lives in industries related to aviation and, of course, know the theory of aeronautics, sometimes honestly admit that they do not understand how airplanes fly. But we will still try to figure it out.

The plane stays in the air thanks to the “lifting force” acting on it, which arises only in the movement provided by the engines mounted on the wings or fuselage.

Jet engines throw back a stream of combustion products of kerosene or other aviation fuel, pushing the plane forward.
The blades of the propeller engine seem to be screwed into the air and pull the plane along with them.

Lifting force

Lift occurs when the oncoming air flows around the wing. Due to the special shape of the wing section, part of the flow above the wing has a higher speed than the flow under the wing. This happens because the upper surface of the wing is convex, as opposed to the flat lower surface. As a result, the air flowing around the wing from above has to travel a longer distance and, accordingly, at a higher speed. And the higher the flow speed, the lower the pressure in it, and vice versa. The lower the speed, the greater the pressure.

In 1838, when aerodynamics as such did not yet exist, the Swiss physicist Daniel Bernoulli described this phenomenon, formulating a law named after him. Bernoulli, however, described the flow of fluid flows, but with the emergence and development of aviation, his discovery could not have come at a more opportune time. The pressure under the wing exceeds the pressure above and pushes the wing, and with it the plane, upward.

Another component of lift is the so-called “angle of attack”. The wing is located at an acute angle to the oncoming air flow, due to which the pressure under the wing is higher than above.

How fast do planes fly?

To generate lift, a certain, and quite high, speed is required. There is a minimum speed, which is necessary for lifting off the ground, a maximum speed, and a cruising speed, at which the plane flies. most route, it is about 80% of the maximum. Cruising speed of modern passenger airliners 850-950 km per hour.

There is also the concept of ground speed, which consists of the aircraft’s own speed and the speed of the air currents that it has to overcome. It is on this basis that the flight duration is calculated.

The speed required for takeoff depends on the weight of the aircraft, and for modern passenger ships ranges from 180 to 280 km per hour. Landing is carried out at approximately the same speed.

Height

The flight altitude is also not chosen arbitrarily, but is determined by a large number of factors, fuel economy and safety considerations.

The air near the surface of the earth is more dense, and accordingly, it has greater resistance to movement, causing increased fuel consumption. As altitude increases, the air becomes more rarefied and resistance decreases. The optimal altitude for flight is considered to be about 10,000 meters. Fuel consumption is minimal.

Another significant advantage of flying at high altitudes is the absence of birds, collisions with which have more than once led to disasters.

Civil aircraft cannot fly above 12,000-13,000 meters, since too much vacuum prevents the normal operation of the engines.

Airplane control

The aircraft is controlled by increasing or decreasing engine thrust. In this case, the speed changes, respectively, the lifting force and flight altitude. For more precise control of the processes of changing altitude and turning, wing mechanization devices and rudders located on the tail unit are used.

Takeoff and landing

In order for the lift to become sufficient to lift the aircraft off the ground, it must develop sufficient speed. This is what runways are used for. For heavy passenger or transport aircraft we need long runways, 3-4 kilometers long.

The condition of the strips is carefully monitored airfield services, keeping them in perfectly clean condition, since foreign objects entering the engine can lead to an accident, and snow and ice on the runway pose a great danger during takeoff and landing.

As the plane takes off, there comes a moment after which it is no longer possible to cancel the takeoff, since the speed becomes so high that the plane will no longer be able to stop within the runway. This is called “speed of decision making”.

Landing is a very crucial moment of flight; pilots gradually slow down, as a result of which lift decreases and the plane descends. Just before the ground, the speed is already so low that flaps are extended on the wings, which slightly increase the lift and allow the plane to land softly.

Thus, no matter how strange it may seem to us, airplanes fly, and in strict accordance with the laws of physics.

The opportunity to fly has always attracted people, but the creation of devices similar to modern airliners just over a hundred years ago seemed absurd. American-born astronomer Simon Newcomb was even credited with mathematical proof that it would not be possible to lift heavier-than-air technology into the sky, but now 11,000-13,000 ships take off every day. We tell you what has changed and how planes manage to transport millions of passengers.

What does flight look like from a physics point of view?

To take off, the device needs to compensate for the force of gravity due to lift and resist the force of air resistance with thrust.

The impossible flight of modern airliners, according to Newcomb's mathematical calculations, can be explained by simple experience. For it you will need 2 identical jars, a pair of similar flies and scales. Place a container with an insect on one bowl, which sits motionless at the bottom. On the other there is a jar with a constantly flying fly.

Logically, the first bowl should outweigh the virtually empty second container. But in reality, both parts of the measure will be in balance. The flying fly is lifted into the air by the downward flow of momentum, adding a few grams to the jar and balancing the force of gravity.

In the case of an airplane, the principle is broadly similar, only the organization is much more complicated. Vehicles fly thanks to the lift force (LF), which arises from the interaction of air flows and the wing with the aerodynamic shape. The latter are located at an angle. With their tip, they cut the flow into a downward and “oncoming” one, which is why an area of high pressure is formed under the wing, and low pressure above it. The difference ultimately generates lift.

But in order to take off, the device needs to compensate not only for the force of gravity due to lift, but also to resist the force of air resistance with thrust. Unlike insects, the ship is not able to gain the required speed and altitude by flapping its wings. The plane will be able to “take off” at a certain speed, which the engines help to reach.

A visual explanation of how and why airplanes fly. What role do the wing, engine and other parts of the structure play in moving through the air?

Take-off and flight speed

The speed (V) of movement of airliners is not constant - one is needed when ascending, and another during flight.

Takeoff actually begins from the moment the vessel moves along the runway. The device accelerates, picks up the pace necessary to lift off from the canvas, and only then, thanks to the increase in lifting force, does it soar upward. The V required for tearing is specified in the manual for each model and general instructions. At this moment, the motors are working at full capacity, putting a huge load on the machine, which is why the process is considered one of the most difficult and dangerous.
In order to fix in space and occupy a designated echelon, it is necessary to achieve a different speed. Flight in a horizontal plane is possible only if the PS compensates for the Earth's gravity.

Indicators of the speed with which an aircraft is able to take off into the air and stay there for certain time, it’s difficult to name. They depend on the characteristics of a particular machine and environmental conditions. A small single-engine V will logically be lower than a giant passenger ship - the larger the vehicle, the faster it has to move.

For a Boeing 747-300, this is approximately 250 kilometers per hour if the air density is 1.2 kilograms per cubic meter. For the Cessna 172 it is approximately 100. The Yak-40 lifts off the road at 180 km/h, the Tu154M at 210. For the Il 96 the average figure reaches 250, and for the Airbus A380 - 268.

Of the conditions independent of the model of the apparatus, when determining the number they rely on:

direction and strength of the wind - the oncoming one helps by pushing the nose up
the presence of precipitation and air humidity - can complicate or facilitate acceleration
human factor - after assessing all parameters, the decision is made by the pilot

Speed typical for the echelon, in technical specifications designated as “cruising” - this is 80% of the maximum capabilities of the vehicle

The speed at the flight level itself also depends directly on the model of the vessel. In the technical specifications it is designated as “cruising” - this is 80% of the maximum capabilities of the vehicle. The first passenger "Ilya Muromets" accelerated to only 105 kilometers per hour. Now the number is on average 7 times higher.

If you fly on an Airbus A220, the indicator is at 870 km/h. The A310 usually moves at a speed of 860 kilometers per hour, the A320 - 840, the A330 - 871, the A340-500 - 881, the A350 - 903, and the giant A380 - 900. Boeing is about the same. Boeing 717 flies at a cruising speed of 810 kilometers per hour. The mass-produced 737 is 817-852 depending on the generation, the long-haul 747 is 950, the 757 is 850 km/h, the first transatlantic 767 is 851, the Triple Seven is 905, and the jet passenger 787 is 902. According to rumors, the company is developing the airliner For civil aviation, which will deliver people from one point to another at V=5000. But for now, the top fastest in the world includes only military personnel:

The American supersonic F-4 Phantom II, although it has given way to more modern ones, is still in the top ten with an indicator of 2370 kilometers per hour
single-engine fighter Convair F-106 Delta Dart with a speed of 2450 km/h
combat MiG-31 - 2993
experimental E-152, whose design formed the basis of the MiG-25 - 3030
prototype XB-70 Valkyrie - 3,308
research Bell X-2 Starbuster - 3,370
The MiG-25 is capable of reaching 3492, but it is impossible to stop at this mark without damaging the engine
SR-71 Blackbird - 3540
world leader X-15 with rocket engine - 7,274

Perhaps civil ships someday they will be able to achieve these figures. But definitely not in the near future, while the main factor in the matter remains the safety of passengers.

4 airliner parts that affect flight performance

Flying cars differ from ordinary ones because they have very complex designs that take into account every little detail. And besides the obvious details, other parts also influence the capabilities and characteristics of movement - in total, 4 main ones were assembled.

1. Wing. If, in the event of an engine failure, you can fly to the nearest airfield on the second one, and if there are problems in two at once, you can land with the experience of a pilot, without a wing you will not be far from the point of departure. Without it, there will be no necessary lifting force. It is no coincidence that they speak about the wing in the singular. Contrary to popular belief, the aircraft has only one. This concept refers to the entire plane diverging in both directions from the side.

Since this is the main part responsible for being in the air, a lot of attention is paid to its design. The form is built according to precise calculations, verified and tested. In addition, the wing is able to withstand enormous loads so as not to jeopardize the main thing - the safety of people.

2. Flaps and slats. Large quantity Over time, an airplane wing has a streamlined shape, but during takeoff and landing additional surfaces appear on it. Flaps and slats are produced in order to increase the area and cope with the forces acting on the vehicle during severe loads at the beginning and end of the journey. When landing, they slow down the plane, do not allow it to fall too quickly, and when ascending, they help to stay in the air.

An airplane is a heavier-than-air aircraft. This means that its flight requires certain conditions, a combination of precisely calculated factors. The flight of an airplane is the result of the lifting force that occurs when air flows move towards the wing. It is turned at a precisely calculated angle and has an aerodynamic shape, thanks to which at a certain speed it begins to strive upward, as the pilots say - “stands up in the air.”

The engines accelerate the plane and maintain its speed. Jet engines push the plane forward due to the combustion of kerosene and the flow of gases escaping from the nozzle with great force. Propeller engines “pull” the aircraft along with them.

The wing of modern aircraft is a static structure and cannot itself generate lift on its own. The ability to lift a multi-ton vehicle into the air occurs only after the forward motion (acceleration) of the aircraft using a power plant. In this case, the wing, placed at an acute angle to the direction of the air flow, creates different pressure: above the iron plate it will be less, and below the product it will be more. It is the pressure difference that leads to the emergence of an aerodynamic force that contributes to the climb.

Aircraft lift consists of the following factors:

Angle of attack
Asymmetrical wing profile

The inclination of the metal plate (wing) to the air flow is usually called the angle of attack. Typically, when lifting an aircraft, the mentioned value does not exceed 3-5°, which is enough for takeoff of most aircraft models. The fact is that the design of the wings has undergone major changes since the creation of the first aircraft and today it is an asymmetrical profile with a more convex top sheet of metal. The bottom sheet of the product is characterized by a flat surface for virtually unhindered passage of air flow.

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Schematically, the process of generating lift looks like this: the upper streams of air need to travel a longer distance (due to the convex shape of the wing) than the lower ones, while the amount of air behind the plate must remain the same. As a result, the upper jets will move faster, creating an area of low pressure according to Bernoulli's equation. The difference in pressure above and below the wing, coupled with the operation of the engines, helps the aircraft gain the required altitude. It should be remembered that the value of the angle of attack should not exceed a critical point, otherwise the lift force will drop.

The wing and engines are not enough for a controlled, safe and comfortable flight. The plane needs to be controlled, and precision control is most needed during landing. Pilots call landing a controlled fall—the plane's speed is reduced so that it begins to lose altitude. At a certain speed, this fall can be very smooth, leading to the wheels of the chassis softly touching the strip.

Flying an airplane is completely different from driving a car. The pilot's control wheel is designed to deflect up and down and create a roll. “Pulling” is a climb. “From yourself” is a decline, a dive. In order to turn or change course, you need to press one of the pedals and use the steering wheel to tilt the plane in the direction of the turn... By the way, in the language of pilots this is called a “turn” or “turn”.

To turn and stabilize the flight, a vertical fin is located at the tail of the aircraft. And the small “wings” located below and above it are horizontal stabilizers that do not allow the huge machine to rise and fall uncontrollably. The stabilizers have movable planes for control - elevators.

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To control the engines, there are levers between the pilots' seats; during takeoff, they are moved fully forward, to maximum thrust, this is the takeoff mode necessary to gain takeoff speed. When landing, the levers are retracted completely back - to the minimum thrust mode.

Many passengers watch with interest as the back of the huge wing suddenly drops down before landing. These are flaps, “mechanization” of the wing, which performs several tasks. When descending, the fully extended mechanization brakes the aircraft to prevent it from accelerating too much. When landing, when the speed is very low, the flaps create additional lift for a smooth loss of altitude. During takeoff, they help the main wing keep the car in the air.

What should you not be afraid of while flying?

There are several aspects of flight that can frighten a passenger - turbulence, passing through clouds and clearly visible vibrations of the wing panels. But this is not at all dangerous - the design of the aircraft is designed to withstand enormous loads, much greater than those that arise during a bumpy ride. The shaking of the consoles should be taken calmly - this is acceptable design flexibility, and flight in the clouds is ensured by instruments.